25K Lines, 2 Weeks, Zero Regressions: The AI-Assisted Migration Methodology That Actually Works

25K Lines, 2 Weeks, Zero Regressions: The AI-Assisted Migration Methodology That Actually Works If you're sitting on a Terraform migration or a K8s API version upgrade that keeps getting pushed to "next quarter" — this might change your math. On February 23, Andreas Kling ported LibJS, Ladybird's entire JavaScript engine frontend, from C++ to Rust using AI coding agents. 25,000 lines. Two weeks. Zero regressions across both the ECMAScript test suite and Ladybird's internal tests. I've read his writeup three times now. The headline is impressive. The methodology is the part worth stealing. What Actually Got Ported LibJS handles the JavaScript engine's lexer, parser, AST (abstract syntax tree), and bytecode generator. This is not a utility library. It's the part of the codebase where a subtle bug can break thousands of programs in ways that don't surface immediately. The kind of code where experienced engineers move carefully and budget months, not weeks, for a rewrite. Kling was using C